Described below is a method for generation or processing of OFDM symbols and a communication system device to execute such a method.
With modern communication systems, especially those which comply with the GSM (Global System for Mobile Communications) or UMTS (Universal Mobile Telecommunications System) Standards, the user data is encoded and divided up on a carrier before a send signal is generated.
With fourth-generation communication systems multiple access methods with OFDM transmission systems (OFDM: Orthogonal Frequency Division Multiplex) are planned. These assign the user data or data from specific data sources in each case to a number of subcarriers in OFDM symbols (OFDM-TDMA) or to a number of different subcarriers (OFDM-FDMA). With OFDM-FDMA (FDMA: Frequency Division Multiple Access) for example the subcarriers represent individual adjacent frequency bands of a larger frequency range in each case. Also known for example is the process of exclusively assigning the user data to a number of complete OFDM symbols in each case, as with OFDM-TDMA (TDMA: Time Division Multiple Access), where the user data in the OFDM symbol for transmission over the interface between different communication stations is assigned in a consecutive sequence, especially user data of a station assigned in each case to one or more of the OFDM symbols directly following each other. Such OFDM transmission methods avoid Inter-Symbol Interferences (ISI).
Furthermore, Multiple Access Interferences (MAI) are avoided with both these multiple access methods.
In accordance with an alternate procedure, by using orthogonal codes with the aid of an orthogonal matrix, the user data can be spread over a specified number of subcarriers and/or OFDM symbols, as with the methods known under OFDM-CDMA or MC-CDMA (CDMA: Code Division Multiple Access; MC: Multiple Carrier). With the OFDM-CDMA method the data of all users is assigned to all available frequencies, in which case codes are used for separation. Without further measures, undesired multiple access interferences occur in this case and especially when considering a number of users or data sources for which the data enters in parallel.
To avoid this problem the user data is thus only spread over a number of subcarriers for simultaneous assignment of subcarriers to a number of users, which is referred to as OFDM-FDMA with user-specific spreading of payload data. Spreading and assignment of subcarriers is undertaken here in a common and comprehensive procedure step. What are known as Walsh-Hadamard Matrices of different sizes are predominantly used for spreading, as known for example from M. Yee, J.-P. Linnartz: Controlled Equalization of Multi-Carrier CDMA in an Indoor Rician Fading Channel, Proc. IEEE VTC'94, Stockholm, Sweden, 1994, or T. Müller, K. Bruninghaus, H. Rohling: Performance of Coherent OFDM-CDMA for Broadband Mobile Communications, Wireless Personal Communications, Kluwer Academic Publisher, 1996. In such systems different users, depending on the data rate required, are assigned a number between 1 and N of the orthogonal codes, with N being the number of available subcarriers.
Furthermore, K. Bruninghaus, H. Rohling: Multi Carrier Spread Spectrum and its Relationship to Single-Carrier Transmission, Proc. IEEE VTC '98, Ottawa, channel, 1998, contains a general discussion of spreading using an FFT matrix (FFT: Fast Fourier Transformation) to reduce the ratio of peak values to average values (PAR: Peak-to-Average-Ratios), in which case multiple accesses are not taken into account however.